分析Python Django源码的运行过程

Django运行顺序

• WSGI会不断监听客户端发送来的请求

• 先经过中间件进行分析验证处理

• 然后经过url分发与验证

• 视图层进行处理

• 再经过中间件进行分析验证处理

• 返回响应内容

1.启动

1.1 命令行启动(测试服务器)

命令行结论：其在第二步utility.execute()函数会根据命令行参数，分发给不同的类进行处理

在manange.py里面execute_from_command_line(sys.argv)进入关键代码

def main():
    os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'testDjango.settings')
    try:
        from django.core.management import execute_from_command_line
    except ImportError as exc:
		---
    execute_from_command_line(sys.argv)

2.execute_from_command_line函数里面其实例化 ManagementUtility类然后执行utility.execute()函数 [
2.1. 此函数是专门用来分析参数的，例如python manage.py runserver、python manage.py help
2.2 其会通过分析额外添加的参数选择要使用的类或者函数，类或者函数对应着django\core\management\commands里面的类

def execute_from_command_line(argv=None):
    utility = ManagementUtility(argv)
    utility.execute()

3.从self.fetch_command(subcommand).run_from_argv(self.argv)[约第413行]
3.1 self.fetch_command(subcommand)，这个函数返回了runserver.Command对象（可以自行深入查看），之后执行该Command父类里面的run_from_argv函数

 def execute(self):
			---
        if subcommand == 'help':
			---
        elif subcommand == 'version' or self.argv[1:] == ['--version']:
            sys.stdout.write(django.get_version() + '\n')
        elif self.argv[1:] in (['--help'], ['-h']):
            sys.stdout.write(self.main_help_text() + '\n')
        else:
            self.fetch_command(subcommand).run_from_argv(self.argv)

4.从run_from_argv函数self.execute(*args, **cmd_options)进入
4.1 当前类也有这个execute函数，但是由于继承关系（此时的self也指向Command类），子类如果已经存在该函数会覆盖执行，execute是在子类 Command类中（之后由于super还会到父类里面）[约第354行]

    def run_from_argv(self, argv):
        self._called_from_command_line = True
        parser = self.create_parser(argv[0], argv[1])
        options = parser.parse_args(argv[2:])
        cmd_options = vars(options)

        args = cmd_options.pop('args', ())
        handle_default_options(options)
        try:
            self.execute(*args, **cmd_options)
        except CommandError as e:
			---

5.execute函数执行output = self.handle(*args, **options)[约第398行]跳进子类runserver.Command类的handle函数
5.1 此时位于Command类的父类里面的execute，因为super().execute(*args, **options) #继承下来父类

    def handle(self, *args, **options):
        if not settings.DEBUG and not settings.ALLOWED_HOSTS:
            raise CommandError('You must set settings.ALLOWED_HOSTS if DEBUG is False.')
        self.use_ipv6 = options['use_ipv6']
        if self.use_ipv6 and not socket.has_ipv6:
            raise CommandError('Your Python does not support IPv6.')
        self._raw_ipv6 = False
        if not options['addrport']:
        	---
        else:
           	---
        if not self.addr:
            self.addr = self.default_addr_ipv6 if self.use_ipv6 else self.default_addr
            self._raw_ipv6 = self.use_ipv6
        self.run(**options)

6.handle 函数最后一行，从 self.run(**options) 进入

    def run(self, **options):
        use_reloader = options['use_reloader']

        if use_reloader:
            autoreload.run_with_reloader(self.inner_run, **options)
        else:
            self.inner_run(None, **options)

7.从def inner_run(self, *args, \*\*options)再执行run函数

    def inner_run(self, *args, **options):
    		---
        try:
            handler = self.get_handler(*args, **options)
            run(self.addr, int(self.port), handler,
                ipv6=self.use_ipv6, threading=threading, server_cls=self.server_cls)
        except OSError as e:
          	 ---

8.最后启动服务，此时跳到django.core.servers.basehttp.py的run函数
8.1 httpd_cls = type('WSGIServer', (socketserver.ThreadingMixIn, server_cls), {}) 这一步特别重要，其涉及到较长的继承关系，2.监听-4.1这一环节会介绍到

def run(addr, port, wsgi_handler, ipv6=False, threading=False, server_cls=WSGIServer):
    server_address = (addr, port)
    if threading:
        httpd_cls = type('WSGIServer', (socketserver.ThreadingMixIn, server_cls), {})
    else:
        httpd_cls = server_cls
    httpd = httpd_cls(server_address, WSGIRequestHandler, ipv6=ipv6)
    if threading:
        httpd.daemon_threads = True
    httpd.set_app(wsgi_handler)
    httpd.serve_forever()

总结流程：

• 解析运行 python manage.py 所提供的参数，例如: help

• 加载所有的app

• 根据参数找到相对应的命令管理工具

• 检查端口、ipv4检测、ipv6检测、端口是否占用、线程检查

• orm对象检查表是否创建

• 最后启动python Lib库中的WSGIServer

2.监听

解释：WSGI开启后，不间断的监听外界的请求

快速阅读：下面写的比较麻烦，最快了解监听和到中间件前的经过就是去读 1 、12.1 和 13

2.1 runserver(测试服务器)

1.runserver成功开启后，关键的一步是httpd.serve_forever()，其使得进入监听即一个死循环

def run(addr, port, wsgi_handler, ipv6=False, threading=False, server_cls=WSGIServer):
	---
    httpd.set_app(wsgi_handler)
    httpd.serve_forever()

2.在serve_forever()函数里面执行，当ready有值时，表示有请求发来，然后进入self._handle_request_noblock()

    def serve_forever(self, poll_interval=0.5):
        self.__is_shut_down.clear()
        try:
            with _ServerSelector() as selector:
                selector.register(self, selectors.EVENT_READ)
                while not self.__shutdown_request:
                    ready = selector.select(poll_interval)
                    if self.__shutdown_request:
                        break
                    if ready:
                        self._handle_request_noblock()

                    self.service_actions()
		---

3.从self._handle_request_noblock()正常请求将进入self.process_request(request, client_address)

    def _handle_request_noblock(self):
        try:
            request, client_address = self.get_request()
        except OSError:
            return
        if self.verify_request(request, client_address):
            try:
                self.process_request(request, client_address)
            except Exception:
                self.handle_error(request, client_address)
                self.shutdown_request(request)
            except:
                self.shutdown_request(request)
                raise
        else:
            self.shutdown_request(request)

4.从self.process_request(request, client_address)进入来到了ThreadingMixIn.process_request
4.1 此时，如果没有搞清楚此时的self是谁，就搞不明白为什么进入到ThreadingMixIn.process_request，而不是其它的process_request，这时候就关联到上面提到的httpd_cls = type('WSGIServer', (socketserver.ThreadingMixIn, server_cls), {})
4.2 type的用法是动态的创建类，此时httpd_cls 是一个新类，里面分别继承了ThreadingMixIn和server_cls对应得WSGIServer，这时就不难理解为什么找的是ThreadingMixIn.process_request

    def process_request(self, request, client_address):
        """Start a new thread to process the request."""
        t = threading.Thread(target = self.process_request_thread,
                             args = (request, client_address))
        t.daemon = self.daemon_threads
        if not t.daemon and self.block_on_close:
            if self._threads is None:
                self._threads = []
            self._threads.append(t)
        t.start()

5.在def process_request(self, request, client_address)里面的t = threading.Thread(target = self.process_request_thread,args = (request, client_address))实际调用了self.process_request_thread，但是等t.start()才会真正执行

    def process_request_thread(self, request, client_address):
        """Same as in BaseServer but as a thread.

        In addition, exception handling is done here.

        """
        try:
            self.finish_request(request, client_address)
        except Exception:
            self.handle_error(request, client_address)
        finally:
            self.shutdown_request(request)

6.从def process_request_thread(self, request, client_address)进入，self.finish_request(request, client_address)，继续完成请求
6.1 这时候又需要回顾之前的代码，因为self.RequestHandlerClass不是已经有的类，而是初始化的时候赋值，其值变为了某个类
6.2 这个过程就在1.启动-8里面的httpd = httpd_cls(server_address, WSGIRequestHandler, ipv6=ipv6)，此时的httpd_cls是type动态创建的，继承了ThreadingMixIn和server_cls对应得WSGIServer，实例化时会执行def __init__方法，其关键执行了self.RequestHandlerClass = RequestHandlerClass

class BaseServer:
    timeout = None

    def __init__(self, server_address, RequestHandlerClass):
        """Constructor.  May be extended, do not override."""
        self.server_address = server_address
        self.RequestHandlerClass = RequestHandlerClass
        self.__is_shut_down = threading.Event()
        self.__shutdown_request = False

    def finish_request(self, request, client_address):
        self.RequestHandlerClass(request, client_address, self)
        # self.RequestHandlerClass等同于self.WSGIRequestHandler

7.从self.RequestHandlerClass(request, client_address, self)，即去WSGIRequestHandler类里面初始化，根据一层层继承关系，只要最老类BaseRequestHandler有初始化方法

class BaseRequestHandler:
    def __init__(self, request, client_address, server):
        self.request = request
        self.client_address = client_address
        self.server = server
        self.setup()
        try:
            self.handle()
        finally:
            self.finish()

从def __init__(self, request, client_address, server):进入self.handle()
8.1 此时的self.handle()，根据继承关系，其就在最小子类WSGIRequestHandler里面

    def handle(self):
        self.close_connection = True
        self.handle_one_request()
        while not self.close_connection:
            self.handle_one_request()
        try:
            self.connection.shutdown(socket.SHUT_WR)
        except (AttributeError, OSError):
            pass

9.从def handle(self)进入self.handle_one_request()

    def handle_one_request(self):
        """Copy of WSGIRequestHandler.handle() but with different ServerHandler"""
        self.raw_requestline = self.rfile.readline(65537)
        if len(self.raw_requestline) > 65536:
            self.requestline = ''
            self.request_version = ''
            self.command = ''
            self.send_error(414)
            return

        if not self.parse_request():  # An error code has been sent, just exit
            return

        handler = ServerHandler(
            self.rfile, self.wfile, self.get_stderr(), self.get_environ()
        )
        handler.request_handler = self      # backpointer for logging & connection closing
        handler.run(self.server.get_app())

10.从def handle_one_request(self)进入handler.run(self.server.get_app())
10.1 注意此时handler为ServerHandler实例化对象，run方法存在它的最大父类BaseHandler里面
10.2 此时handler.run(self.server.get_app())执行了self.server.get_app()，其返回django.contrib.staticfiles.handlers.StaticFilesHandler，handler.run把其当参数传递了过去

    def run(self, application):
        try:
            self.setup_environ()
            self.result = application(self.environ, self.start_response)
            self.finish_response()
        except (ConnectionAbortedError, BrokenPipeError, ConnectionResetError):

            return
        except:
			---

11.从def run(self, application)进入self.result = application(self.environ, self.start_response)，其中application是django.contrib.staticfiles.handlers.StaticFilesHandler
11.1 其中self.application已经初始化了是WSGIHandler

class StaticFilesHandler(StaticFilesHandlerMixin, WSGIHandler):
    def __init__(self, application):
        self.application = application
        self.base_url = urlparse(self.get_base_url())
        super().__init__()

    def __call__(self, environ, start_response):
        if not self._should_handle(get_path_info(environ)):
            return self.application(environ, start_response)
        return super().__call__(environ, start_response)

12.进入后执行def __call__(self, environ, start_response)方法，进入return self.application(environ, start_response)，此时self.application已经初始化了是WSGIHandler
12.1 request = self.request_class(environ)获取到用户请求的url后面就开始配置runserver启动时候加载的url； response = self.get_response(request)获取用户url对应的响应准备开始往视图转

    def __call__(self, environ, start_response):
        set_script_prefix(get_script_name(environ))
        signals.request_started.send(sender=self.__class__, environ=environ)
        request = self.request_class(environ)
        response = self.get_response(request)
		---

13.进入response = self.get_response(request)，结束，再下一步就要开始中间件的进行

    def get_response(self, request):
        set_urlconf(settings.ROOT_URLCONF)
        response = self._middleware_chain(request)
        response._resource_closers.append(request.close)
        if response.status_code >= 400:
            log_response(
                '%s: %s', response.reason_phrase, request.path,
                response=response,
                request=request,
            )
        return response

3.中间件的执行

解释：中间件的执行需要联系着上面运行过程，这个过程是一个递归的过程，下面介绍的五个函数是中间件命名规则对应得内容

• process_request()：完成请求对象的创建，但用户访问的网址尚未与网站的路由地址匹配。

• process_view()：完成用户访问的网址与路由地址的匹配，但尚未执行视图函数。

• process_exception()：在执行视图函数的期间发生异常，比如代码异常，主动抛出404异常等。

• process_response()：完成视图函数的执行，但尚未将响应内容返回浏览器

• process_template_response()：默认不执行，在视图函数完成操作后调用，除非视图函数返回的response中有render方法(几乎不会用，可以忽略)

1.递归的进入阶段：循环进行下面的代码（此代码位置django\core\handlers\exception.py）

1.1 此处出现process_request()和process_response()

        @wraps(get_response)
        def inner(request):
            try:
                response = get_response(request) # 此进入循环
            except Exception as exc:
                response = response_for_exception(request, exc)
            return response
        return inner

    def __call__(self, request):
        # Exit out to async mode, if needed
        if asyncio.iscoroutinefunction(self.get_response):
            return self.__acall__(request)
        response = None
        if hasattr(self, 'process_request'):
            response = self.process_request(request) # 进行中间件的process_request步骤
        response = response or self.get_response(request) # 此进入循环
        if hasattr(self, 'process_response'):
            response = self.process_response(request, response) # 此是递归后执行的
        return response

2.递归的结束准备回传：进行下面的代码（此代码位置django\core\handlers\base.py）
2.1 此处出现process_view()、process_template_response()和process_exception()

进入视图的关键函数：

• callback, callback_args, callback_kwargs = self.resolve_request(request) # callback即对于视图函数url匹配到对应的view函数

• for middleware_method in self._view_middleware_view_middleware里面放着所有的process_view()函数（初始化时加载的）， process_view()正是在该代码下面的环节循环执行

• response = wrapped_callback(request, *callback_args, **callback_kwargs) 回调函数传参，并返回试图函数响应。

• 沿着这个路径连续进入两次，就到了后面讲到的as_view里面（此内容是专门视图处理的前的关键步骤）

• response = self.process_exception_by_middleware(e, request)对应process_exception()

• self._template_response_middleware 循环加载模板中间件

 def _get_response(self, request):
        response = None
        callback, callback_args, callback_kwargs = self.resolve_request(request)
        for middleware_method in self._view_middleware:
            response = middleware_method(request, callback, callback_args, callback_kwargs)
            if response:
                break
        if response is None:
            wrapped_callback = self.make_view_atomic(callback) # 找到视图函数
            # If it is an asynchronous view, run it in a subthread.
            if asyncio.iscoroutinefunction(wrapped_callback):
                wrapped_callback = async_to_sync(wrapped_callback)
            try:
                response = wrapped_callback(request, *callback_args, **callback_kwargs)
            except Exception as e:
                response = self.process_exception_by_middleware(e, request)
                if response is None:
                    raise
        self.check_response(response, callback)
        if hasattr(response, 'render') and callable(response.render):
            for middleware_method in self._template_response_middleware:
                response = middleware_method(request, response)
                self.check_response(
                    response,
                    middleware_method,
                    name='%s.process_template_response' % (
                        middleware_method.__self__.__class__.__name__,
                    )
                )
            try:
                response = response.render()
            except Exception as e:
                response = self.process_exception_by_middleware(e, request)
                if response is None:
                    raise
        return response

3.递归的结束回传：循环进行下面的代码

        @wraps(get_response)
        def inner(request):
            try:
                response = get_response(request) # 此进入循环
            except Exception as exc:
                response = response_for_exception(request, exc)
            return response
        return inner

    def __call__(self, request):
        # Exit out to async mode, if needed
        if asyncio.iscoroutinefunction(self.get_response):
            return self.__acall__(request)
        response = None
        if hasattr(self, 'process_request'):
            response = self.process_request(request) 
        response = response or self.get_response(request) # 此进入循环
        if hasattr(self, 'process_response'):
            response = self.process_response(request, response) # 进行中间件的process_response步骤
        return response

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